TB Skin Test

Tests and diagnosis

During the physical exam, your doctor will check your lymph nodes for swelling and use a stethoscope to listen carefully to the sounds your lungs make while you breathe.

The most commonly used diagnostic tool for tuberculosis is a simple skin test, though blood tests are becoming more commonplace. A small amount of a substance called PPD tuberculin is injected just below the skin of your inside forearm. You should feel only a slight needle prick.

Within 48 to 72 hours, a health care professional will check your arm for swelling at the injection site. A hard, raised red bump means you’re likely to have TB infection. The size of the bump determines whether the test results are significant.

Results can be wrong

The TB skin test isn’t perfect. Sometimes, it suggests that people have TB when they really don’t. It can also indicate that people don’t have TB when they really do.

A false-positive test may happen if you’ve been vaccinated recently with the bacille Calmette-Guerin (BCG) vaccine. This tuberculosis vaccine is seldom used in the United States but is widely used in countries with high TB infection rates.

False-negative results may occur in certain populations — including children, older people and people with AIDS — who sometimes don’t respond to the TB skin test. A false-negative result can also occur in people who’ve recently been infected with TB, but whose immune systems haven’t yet reacted to the bacteria.

Blood tests

Blood tests may be used to confirm or rule out latent or active tuberculosis. These tests use sophisticated technology to measure your immune system’s reaction to TB bacteria. QuantiFERON-TB Gold in-Tube test and T-Spot.TB test are two examples of TB blood tests.

These tests require only one office visit. A blood test may be useful if you’re at high risk of TB infection, but have a negative response to the skin test, or if you’ve recently received the BCG vaccine.

Imaging tests

If you’ve had a positive skin test, your doctor is likely to order a chest X-ray or a CT scan. This may show white spots in your lungs where your immune system has walled off TB bacteria, or it may reveal changes in your lungs caused by active tuberculosis. CT scans provide more-detailed images than do X-rays.

Sputum tests

If your chest X-ray shows signs of tuberculosis, your doctor may take samples of your sputum — the mucus that comes up when you cough. The samples are tested for TB bacteria.

Sputum samples can also be used to test for drug-resistant strains of TB. This helps your doctor choose the medications that are most likely to work. These tests can take four to eight weeks to be completed.